linux-kernel - Re: [RFC PATCH 3/4] kernel/workqueue: Distinguish between general unbound and WQ

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Message-ID: <72e4a971-96e5-44b7-b348-bbdb68e54b40@redhat.com>
Date: Wed, 17 Jan 2024 14:32:34 -0500
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>, Juri Lelli <juri.lelli@...hat.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>, Aaron Tomlin
 <atomlin@...mlin.com>, Valentin Schneider <vschneid@...hat.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/4] kernel/workqueue: Distinguish between general
 unbound and WQ_SYSFS cpumask changes


On 1/17/24 12:12, Tejun Heo wrote:
> Hello,
>
> On Wed, Jan 17, 2024 at 02:06:08PM +0100, Juri Lelli wrote:
>>> This looks rather hacky. Can you elaborate how the current code misbehaves
>>> with an example?
>> I was trying to address the fact that ordered unbound workqueues didn't
>> seem to reflect unbound_cpumask changes, e.g.
>>
>> wq_unbound_cpumask=00000003
>>
>> edac-poller              ordered,E  0xffffffff 000000ff      kworker/R-edac-            351 0xffffffff 000000ff
>>
>> vs.
>>
>> edac-poller              ordered,E  00000003                 kworker/R-edac-            349 00000003
>>
>> with the patch applied. But honestly, I'm now also not convinced what
>> I'm proposing is correct, so I'll need to think more about it.
>>
>> Can you please confirm though that ordered unbound workqueues are not
>> "special" for some reason and we would like them to follow
>> unbound_cpumask changes as normal ubound workqueues?
> They aren't special and should follow the normal unbound workqueue cpumask.

My impression is that changing the workqueue cpumask of ordered unbound 
workqueue may break the ordering guarantee momentarily. I was planning 
to look into this further to see if that is true when I have time. If it 
is not a concern, we should certainly apply the global unbound cpumask 
change to those workqueues as well.

Cheers,
Longman